1. Field of the Invention
The present invention relates in general to power control for Universal Serial Bus (USB) devices, and more specifically to a system and method of adaptive feedback power control for charging the external voltage of a USB device.
2. Description of the Related Art
The Universal Serial Bus (USB) was designed to allow multiple peripheral devices to be connected to a host computer system. USB uses a single standardized interface socket to improve plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer system. USB has become the standard connection method for many peripheral devices. For example, USB is often used to connect computer peripheral devices such as a computer mouse, keyboard, personal digital assistants (PDAs), gamepads and/or joysticks, scanners, digital cameras, printers, communication ports (e.g., serial, parallel, etc.), personal media players, flash drives, storage drives, mobile phones (e.g., cellular phones and the like), etc. The USB connection may be used for communications or for providing power (such as charging a battery of a portable device) or both. USB was originally designed for connecting peripheral devices to personal computers, but USB has become commonplace on other devices such as PDAs, hand-helds, video game consoles, mobile phones, car stereos, etc. USB is currently described by the USB 2.0 Specification, which includes the earlier USB 1.1 Specification and which introduces a third, higher speed. Future USB Specifications are contemplated for providing greater capabilities and/or higher speeds, such as, for example, the anticipated USB 3.0 Specification.
USB On-The-Go (OTG) is a supplement to the USB Specification 2.0 which allows two USB devices to communicate without requiring the services of a host computer system. USB-OTG retains the standard USB host and peripheral model in which one host communicates with one or more USB peripheral devices. Some devices, such as personal computers and the like, might be configured only to operate as a host whereas many conventional peripheral devices may be configured only to operate as peripheral devices. OTG introduces a dual-role device (DRD) in which at least one device is capable of functioning either as a host or as a peripheral and the initially assigned or default roles may be swapped. Once connected, OTG DRD devices can exchange host/peripheral roles using a Host Negotiation Protocol (HNP). In certain configurations the cable orientation determines the initial or default roles of coupled devices. If the roles need to be reversed or if the cable orientation is backwards (such as when a peripheral-only device is determined as the initial host device according to the cable orientation), HNP enables the devices' roles to reverse automatically and silently regardless of the cable orientation and without requiring the USB cable to be reversed.
The host device provides power to the peripheral device via the USB cable. A host device or OTG DRD device typically includes an internal power regulator for regulating power supply voltage to device circuitry. The device further includes USB circuitry and a USB port for interfacing external devices via the USB cable. The USB circuitry includes a USB transceiver for communicating to other devices via the USB port and cable. The USB port and cable includes a VBUS signal and a ground signal for providing or receiving power. If the USB device is an OTG DRD device operating as a peripheral device, or if it is a peripheral-only type device, then an external host provides power to the USB transceiver via the USB port. If the USB device is operating as a host device, however, then the device itself provides the power to its transceiver and to external devices via the USB port. It is possible for the host to have two additional regulators (other than the device internal power regulator) including a first regulator for providing power to the internal USB transceiver and a second regulator for providing power to external devices. Such configuration is relatively inefficient and costly. It is desired to use the device internal power regulator to provide power to the internal USB transceiver and to external devices in an efficient and cost-effective manner. Such power configuration should be implemented in such a manner to avoid drop or overshoot of the regulator voltage which might otherwise cause instability and/or unreliable operations of the USB devices.